The present invention relates to a method for curing an epoxy resin with a curing agent. More particularly, the invention relates to a method for curing an epoxy resin with a curing agent which is a polyamide derived from a polyamine and a novel dimeric acid mixture capable of giving a curable epoxy resin composition having a relatively low viscosity and exhibiting good flowability at low temperatures as compared with conventional dimeric acid mixtures.
As is known, epoxy resins are cured by admixing the resin with a curing agent to form a curable epoxy resin composition and keeping the composition at room temperature or an elevated temperature. Various types of curing agents are known including the classes of catalytic curing agents and crosslinking curing agents and practically used. It is also known that the properties of the cured epoxy resin largely depend on the types of the curing agents. In this regard, each of the various curing agents has its own merits and demerits.
As a class of the above mentioned crosslinking curing agents for epoxy resins, polyamides derived from a polyamine and a dimeric acid, i.e. a dimer of an aliphatic carboxylic acid, are sometimes utilized advantageously due to the unique properties of the cured epoxy resin. A problem in the use of such a polyamide as a crosslinking curing agent of an epoxy resin, however, is that the epoxy resin composition after admixture with the curing agent has a relatively high viscosity with low flowability sometimes to cause inconveniences or decrease in the workability, for example, in resin casting.
Dimeric acid is a dibasic carboxylic acid obtained by the intermolecular addition reaction or dimerization between two molecules of an unsaturated fatty acid and is usually obtained as a mixture of several kinds of different molecular species. For example, commercial products of dimeric acid are each a mixture mainly consisting of one or more of dibasic carboxylic acids having 36 carbon atoms in a molecule in admixture with tribasic carboxylic acids having 54 carbon atoms in a molecule as a major secondary constituent.
Such a dimeric acid as a mixture has unique properties that it has high reactivity, it is a dibasic carboxylic acid having the largest molecular weight among those industrially available, it is strongly hydrophobic by virtue of the large hydrocarbon groups in the molecular structure, polymers derived from a dimeric acid are amorphous and flexible and have good mechanical properties at low temperatures and so on so that dimeric acid mixtures are useful in various applications.
In particular, amidation products of such a dimeric acid mixture without reactivity have characteristics of high tenacity along with elastic flexibility as well as strong adhesive power to various substrate surfaces. In addition, they have good miscibility with other kinds of resins such as those of ink vehicles. Therefore, the amidation products are useful as adhesives, additives in printing inks and coating compositions, sealants and the like. Reactive amidation products of a dimeric acid mixture have usefulness as an additive in epoxy resins used for coating of metals, plastics, masonry materials and the like or used as an adhesive of a two-component type.
A typical known method for the preparation of a dimeric acid mixture is that an unsaturated fatty acid is heated at a temperature of 200.degree. to 250.degree. C. in the presence of a catalyst which is usually a montmorillonitic clay to effect dimerization of the acid. In view of enhancement of the velocity of thermal polymerization when the starting unsaturated fatty acid has two trans-double bonds in conjugation, a standard industrial manufacturing process of dimeric acid mixtures has been established by using tall oil fatty acid or soybean oil fatty acid containing a large amount of linoleic acid as the starting material.
The dimeric acid mixture prepared by the above mentioned process, however, contains a relatively large amount of dimeric molecular species having a monocyclic or polycyclic structure, such as those expressed by the formulas ##STR2## as a consequence of the high content of linoleic acid in the starting material so that the dimeric acid mixture has a high viscosity and high pour point to cause inconveniences in the use thereof. In particular, the amidation product obtained by the condensation reaction thereof with an alkylene diamine compound is defective in respect of the high melting point and high melt viscosity.
It is known in the organic chemistry that a hydrocarbon compound having a cyclic molecular structure generally has a higher melting point and higher melt viscosity than the corresponding compound having the same number of carbon atoms in a molecule but having a non-cyclic or straightly linear molecular structure as a consequence of the steric effect by the cyclic structure. It is reported in Journal of American Oil Chemists' Society, volume 51, page 522 (1974) that, when a mixture of unsaturated fatty acids containing 78% by weight of oleic acid is heated at about 250.degree. C. in the presence of a montmorillonite catalyst to effect dimerization, the dimeric acid mixture obtained contains 40% by weight of non-cyclic dimeric acids with a decrease in the content of cyclic dimeric acids.
The polyamide derived from the dimeric acid mixture described above is still disadvantageous as a curing agent of epoxy resins in respect of the relatively high viscosity of the epoxy resin composition compounded therewith. It is of course a possible way that such a high-viscosity epoxy resin composition is diluted with an organic solvent such as xylene or an alcohol although use of an organic solvent causes problems that the epoxy resin after curing may have a decreased impact strength or poor thermal stability due to the trace amount of the residual organic solvent in addition to the serious problem that organic solvents in general are responsible for heavy environmental pollution. Accordingly, it is eagerly desired to develop a method for curing an epoxy resin with admixture of a curing agent, in which the epoxy resin composition after admixture of the curing agent may have a sufficiently low viscosity even without addition of an organic solvent.